This invention relates to implantable devices which detect and/or treat tachyarrhythmias and more specifically to telemetry systems employed in such devices to transmit information to an external receiver to assist the physician in monitoring and understanding the operation of the devices.
In pacemaker/cardioverter/defibrillators presently in clinical evaluation, ventricular fibrillation and ventricular tachycardias are identified using rate based criteria. In such devices, it is common to specify rate or interval ranges that characterize one or more types of ventricular tachycardias and fibrillation. Counts of the measured R--R intervals which fall into the rate ranges are used to determine whether a tachyarrhythmia is present and to diagnose the particular tachyarrhythmia. The detection methodologies practiced in such devices may be difficult for the physician to follow, as the individual intervals may increment or not increment an individual count depending on factors other than the interval duration alone. For example, rapid onset criteria based on preceding intervals may be required to initiate counting and stability criteria based on the preceding intervals may be required to continue counting. In some devices, whether a measured R--R interval increments a count and which count is incremented may be a function of both the individual interval duration and the average rate over the preceding series of intervals. Patents illustrative of such detection methodologies include U.S. Pat. Nos. 4,830,006, 4,969,465 and 5,063,928.
In implantable anti-tachyarrhythmia devices, each of the possible diagnoses provided by the device will trigger a predefined therapy, with the general aggressiveness of the therapies increasing from least aggressive if the diagnosis is a slow ventricular tachycardia to most aggressive if the diagnosis is ventricular fibrillation. For example, anti-tachycardia pacing may be employed in response to a diagnosis of slow ventricular tachycardia, cardioversion may be employed if the diagnosis is fast ventricular tachycardia and defibrillation may be employed if the diagnosis is fibrillation.
Most currently available devices of the type in which the present invention may be practiced provide for a menu of available therapies for each type of tachyarrhythmia diagnosed. In programming such devices it is typical to specify a number of different therapies for each diagnosed tachyarrhythmia, with therapies attempted sequentially in response to the failure of a previous therapy to terminate the arrhythmia. The sequence of therapies actually delivered in such devices may also be difficult for the physician to follow, as the sequence may depend not only on the particular arrhythmia presently diagnosed, but on the success or failure of preceding therapies delivered in response to previously detected arrhythmias and/or on whether the rhythm has accelerated or decelerated during delivery of a preceding therapy. Patents illustrating such therapy delivery schedules and menus include U.S. Pat. Nos. 4,830,006, 4,969,465 and 5,063,928, referred to above.
Presently available implantable anti-tachyarrhythmia devices typically also include provision for storage of diagnostic information such as the numbers and types of arrhythmias detected and of the therapies delivered. Some devices, such as the Medtronic Model 7216 and 7217 implantable pacemaker/cardioverter/defibrillators also have the capability to transmit marker channel information in conjunction with each sensed R-wave. This information may indicate which interval range the interval ending with the sensed R-wave falls into, based on the programmed criteria for identifying the various tachyarrhythmias. Other devices include the ability to store and transmit segments of recorded ECG associated with the detection of arrhythmias and/or the delivery of anti-tachyarrhythmia therapies. Patents disclosing such diagnostic telemetry systems include U.S. Pat. Nos. 4,295,474, 4,223,678 and U.S. Pat. No. 4,567,883.
Recently, a new telemetry format for implantable devices has been developed, which allows for transmission of analog and digital information in more compact form, as disclosed in U.S. Pat. No. 5,127,404, issued to Wyborny et al., incorporated herein by reference in its entirety. This telemetry format employs pulse position modulation to encode data in hexadecimal form rather than binary form, dramatically reducing the time required to transmit substantial blocks of information.